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Dr Sheppeard stressed the importance of adults up to 53 years of age checking their vaccination history before travelling overseas.

"Measles cases worldwide have dramatically increased in 2019, and many countries are experiencing large and ongoing outbreaks. This means that all travellers are at risk of measles if they are unvaccinated, not just those travelling to the usual hotspots," said Dr Sheppeard.

"Two doses of the measles-mumps-rubella vaccine provides lifelong protection in 99 out of 100 people who are vaccinated.

"If you are unsure whether you have had two doses of measles vaccine, it is safe to have another."

NSW Health provides a free measles vaccine for anyone born during or after 1966 who hasn't already had two doses. "Anyone planning a trip overseas should seek medical advice at least two weeks before departure on the vaccinations required to protect them against the risk of disease," she said.

Warnings have also been issued over locations the two people diagnosed yesterday visited back in Australia.

Last Thursday, 15 August, one of the infected people took the train from Hazelbrook to Central arriving at 7.15am, travelling in the last carriage.

One of those affected then travelled by train from Wynyard to Central departing at 5pm and then heading to Hazelbrook at 5.17pm.

The next day they followed a similar routine on the trains and gym before heading to Barangaroo again.

The other person also visited Hornsby Westfield's Seafood@Hornsby restaurant around lunch time on Friday.

On the morning of Monday 19 August one of the patients visited Hazelbrook shops, including IGA and the pharmacy.

Measles outbreak

Pockets of under-vaccinated adolescents and young adults could be putting the community at risk of contracting measles as outbreaks continue, an expert has warned.

The data reflects the growing trend of outbreaks overseas, with the World Health Organisation revealing this week that the numbers infected with measles have tripled worldwide this year.

A total of 170 countries have already reported just over 112,000 measles cases to WHO from January to March.

Why is measles coming back?

Despite measles being officially eradicated in Australia in 2014, the epidemics overseas have led to travellers bringing the highly-infectious disease back home.

With its high rates of vaccination, Australia relies on herd immunity to stop the disease from spreading.

Until now, the focus has been on making sure babies and infants are immunised, but there are other age groups in Australia who could be slipping through the cracks, Raina MacIntyre, a professor of Infectious Diseases Epidemiology at the University of NSW told nine.com.au.

What to know about the measles vaccine

To be fully immunised you need two doses of the measles-mumps-rubella vaccine.

These days, the doses are usually given to infants at 12 months and 18 months.

However, the second dose of the vaccine wasn't routinely given until the 1990s.

A catch-up campaign was introduced in Australian schools in 1992, but those born earlier may have missed out, or not been immunised at all given the rates were much lower then than they are now.

How to get measles

Symptoms of measles include fever, sore eyes and a cough followed three or four days later by a red, blotchy rash.

It is highly infectious and is spread through coughing and sneezing.

"People spread it through the air, with their respiratory secretions. It's one of the most infectious, vaccine preventable diseases that we have. It's far more infectious than smallpox was," Professor MacIntyre told nine.com.au.

Often the disease was spread in the hospital environment when people who had been overseas came back home, became sick and sought help in an emergency department, she said.

"We see this time and time again. A lot of people don't seek travel health advice when they go overseas. They might travel somewhere in Asia, or Europe or the US where there is a lot of measles going on.

"Then they come back and often they might sit in an emergency department for 3-4 hours with a rash and fever.